With the Infinity Stone of Power taken from Xandar, Thanos and his army lay siege to the Asgardian refugees escaping the destruction of their homeworld. Thanos' Black Order present Loki to their master, who demands from the Asgardian the Space Stone. With a massive purple hand clamped around Thor's head, Thanos threatens to kill the God of Thunder if Loki refuses. After a moment of consideration, the God of Mischief encourages the Mad Titan to kill his adoptive brother, who cheerfully obliges.
Placing the Power Stone flush against Thor's temple, the unfiltered energy causes him to scream in agony as Loki watches on, visibly cracking. Just as the surge is about to reach critical mass, Loki breaks and reveals he stole the Space Stone from Odin's vault just before Asgard's destruction. His exact motive — insurance, a desire to be part of something bigger — no longer matters; if Thanos will spare Thor, he can have it. The universe hangs in the balance, and Loki finds himself bargaining for the life of one man.
"You...really are the worst, brother," mutters Thor. However, before Thanos can claim his prize, Loki — ever the master of backup plans — echoes the words of a seldom-seen enemy of his past:
Surprised by an attack he clearly didn't see coming, the Hulk tackles the Mad Titan and beats him into a corner. The Black Order move to help their master, but Ebony Maw stops them and advises them to "let him have his fun."
Thunderous blows echo through the ship's wreckage as the Hulk holds nothing back, and Thanos basically stands and takes it. The novelty wears thin, however, and Thanos quickly recovers and fights back. The Hulk is completely outmatched by Thanos's superior combat skill, and with a few well timed counters, Thanos beats the Hulk into unconsciousness. Thor attempts to intervene but is thrown back by Thanos and bound by Ebony Maw with metal shards from the broken ship. Hulk is only barely spared from a grisly death by Heimdall, who uses what's left of the Bifrost to send the Hulk back to Earth, after which Thanos promptly kills him. Thor screams in anguish and swears his vengeance, but Ebony Maw simply adds a metal gag in response. Thanos eventually takes the Tesseract and breaks it to pieces in his bare hand, takes the Space Stone inside, adding it to his Gauntlet along with the Power Stone.
Thanos orders his lieutenants to Earth to find the two Infinity Stones there and bring them to him; Loki then attempts to placate Thanos with his knowledge of Earth, but the offer is instantly shot down, as he knows only failure. Loki continues to advertise his services, highlighting his accomplishments — while emphasizing his status as an "Odinson" — until he pulls a knife and attempts to stab Thanos, who freezes his arm just shy of his neck using the Space Stone. Unimpressed, the titan wraps a single hand around the Asgardian's neck, lifts him off his feet, and begins to strangle him to death.
Loki, realizing he can't trick his way out of this, spits his last words at his former master,
"You will never be a god!"
Thanos, visibly offended by this, compresses even harder until Loki's neck breaks and drops his limp body onto the ship's floor right in front of Thor. "No resurrections this time" the Mad Titan sneers.
Thanos then channels the Power Stone's energy through the ship before teleporting away with his "children". The bonds fall off Thor and he drags himself over to hold his brother's lifeless body, with his best friend's corpse just meters on the broken wreckage of the colony ship that once carried the last of his people, in his grief not even paying attention to the progressing explosion around him.
Thor is left for dead, the Hulk has been beaten to a pulp, the biggest threat the Avengers ever faced has been murdered, and a genocidal Galactic Conquerer now has two of the six pieces of the universe.
Hulk crash-lands in Manhattan, in the Sanctum Sanctorum, and reverts back to Bruce Banner. Banner is discovered by Doctor Stephen Strange and Wong, overseers of the Sanctum, and gives them grave tidings; Thanos is coming. Strange calls Tony Stark, the Iron Man, in response, and warns him that if Thanos manages to get all six Infinity Stones, the consequences will be dire. Tony knows their best bet is to ally with Captain Steve Rogers, but following the Civil War that had been waged within the Avengers, he hesitates to call for his aid.
Before he can overcome his reservations, two of Thanos's soldiers, Ebony Maw and Cull Obsidian, go on the attack, seeking out Strange and the Time Stone that is kept within one of his artifacts, the Eye of Agamotto. Tony activates a new Iron Man suit made out of nanotechnology that can flash-forge additional weapons. Peter Parker, who sees Maw's ship approach Manhattan, joins the fray as Spider-Man. Banner attempts to help, but the Hulk completely refuses to heed Banner's call, being tired of playing the hero to Bruce Banner. Maw pins the sorcerer down and attempts to rip the necklace away, only to be thwarted by a protection spell.
"It's a simple spell," Strange says. "But quite unbreakable."
"Then I'll take it off your corpse!" Maw replies.
In the end, Wong manages to strand Obsidian in a frozen wasteland, but Strange is beamed aboard Maw's ship. Maw promptly abandons his ally and takes off, hoping to get sole credit for taking the Time Stone. Spider-Man is left dangling from the outside of the ship; as it climbs higher, he nearly suffocates until Stark provides him with a nanotech Iron Spider suit. Both of them (and Strange's Cloak of Levitation) wind up inside the vessel as it leaves Earth behind.
With Strange and the Time Stone off-world, Wong realizes he's now the sole guardian of the New York Sanctum, and leaves the battlefield to protect it and its secrets. Banner, meanwhile, has a call to make...
In Scotland, Vision begins to receive disturbing premonitions from his Mind Stone. Wanda Maximov, the Scarlet Witch, is with him as they experiment with the prospect of a relationship. With one being a fugitive and the other obligated to arrest her, they keep their rendezvous a secret. Vision escorts her to a train-station to part ways, but almost persuades her to stay, convinced that their relationship can work. However, they see the news of Maw and Obsidian's ship, and reports that Iron Man is missing.
Before their conversation can go further, they are both ambushed by two more of Thanos's underlings, Proxima Midnight and Corvus Glaive. Vision is stabbed in the back by Glaive's spear and rendered unable to phase, but he and Wanda are saved by the timely intervention of the Secret Avengers: Captain Steve Rogers; Natalia Romanova, the Black Widow; and Sam Wilson, the Falcon. They meet with James Rhodes, War Machine, Tony's old friend at the Avengers compound, who chooses to stand with them despite Thaddeus Ross's threats, where they learn of Thanos's threat from Banner who is also present. Vision offers to have the Mind Stone destroyed, sacrificing himself to thwart Thanos's plan. Steve, however, shoots down the offer.
"We don't trade lives," he says, firmly.
Instead, he proposes making their way to Wakanda, in the hopes that their allies there can safely remove the Mind Stone without sacrificing Vision.
Out in the depths of space, the Guardians of the Galaxy are responding to the Asgardians' distress signal (as they characteristically sing along to The Rubberband Man on Quill's new Zune) when they literally run into Thor. When he regains consciousness, Thor explains that Thanos is on his way to Knowhere to take the Reality Stone from the Collector. If they are to stand a chance of defeating Thanos, Thor would need a new weapon to replace the destroyed Mjolnir. Thor, Rocket, and Groot make their way to the forge of Nidavellir to retrieve this weapon while Peter "Star-Lord" Quill, Gamora, Drax, and Mantis head to Knowhere to stop Thanos. Along the way, Gamora confides in Quill that she has knowledge Thanos seeks, and makes him swear on his mother that, if she is in danger of being captured, he would kill her to keep that knowledge out of Thanos's grasp.
Quill and his group head to Knowhere, and Gamora attempts to kill Thanos. It seems she has succeeded at first, but in truth, Thanos had already gotten the Reality Stone and used an illusion to lure them out. He uses the Reality Stone to incapacitate Drax and Mantis, then takes Gamora captive. Quill tries to keep his promise and kill Gamora, but Thanos renders his blaster useless and escapes with Gamora back to his flagship, where he is keeping Gamora's sister Nebula, having captured her after Nebula's failed attempt on his life. Thanos had tortured Nebula in the hopes of getting the location of the Soul Stone, and tortures her further until Gamora finally reveals that it is kept on Vormir.
On the way to Nidavellir, Thor confides in Rocket the extent of his recent losses. Despite the odds, however, he remains convinced fate will give him his revenge against Thanos — he sees no other reason for it to have spared him. When Rocket asks what happens if he's wrong, Thor bitterly replies, "what more could I lose?" Upon arrival, Nidavellir is revealed to be a series of giant Dyson rings oscillating a neutron star, but there's a problem — the star is cold. On further investigation, they find the dwarf, Eitri, who explains that Thanos held the dwarves hostage to build the Infinity Gauntlet, promising mercy in return for their services. Because of Hela's usurpation of the Asgardian throne the dwarves couldn't call for help and were forced to bend to the Titan's wishes. Unfortunately, Thanos killed all the dwarves anyway and encased Eitri's hands in metal. Thor tries to reason with the dwarf, pointing out that with his guidance, they can make a weapon powerful enough to kill Thanos — Stormbreaker.
Thanos takes Gamora with him to Vormir, where they encounter the Soul Stone's keeper, the man once known as the Red Skull, transported to Vormir by the Space Stone for his crimes and forced to guard the Soul Stone, a treasure he can never possess. The Red Skull explains that there is a price to be paid to claim the Soul Stone; one must be willing to offer the life of the person they hold most dear in exchange for the Stone. Gamora believes Thanos's plan is doomed to fail, claiming that Thanos cares for nobody other than himself. When she sees Thanos weeping silently, however, she has a harrowing realization; Thanos truly did love her as a daughter, but worse still, he is willing to offer her life for the Stone. Gamora struggles to break free of Thanos's grasp, but to no avail; he tosses her to her death, and takes the Soul Stone.
As Nebula escapes captivity and instructs the Guardians to rendezvous on Thanos's homeworld of Titan, Peter and Tony fight their way through Maw's ship, sending Maw flying into the depths of space and rescuing Strange. Strange wants to take control of the ship and pilot it back to Earth, but Tony isn't sure if they should. Strange balks at the idea of essentially bringing Thanos the Time Stone, but Tony is sick of waiting for the enemy to come to them. Fighting Thanos on his home ground will be risky, but at least it'll be on their terms and away from any civilians. Strange relents, but makes it clear that if he has to put The Needs of the Many before Tony or Peter, he will let them die.
Back at Nidavellir, Thor uses his strength and Rocket's shuttle-pod to force the rings to swing again and the star opens up. The energy is filtered through the rings into the forge and the process begins ... until the main pin in the focusing lens breaks; sabotaged by Thanos. Thor has no choice but to use himself as a last-minute substitute, taking the full brunt of the star.
Iron Man, Doctor Strange and Spider-Man make an extremely rough landing on Titan. Within seconds, they're confronted by the Guardians, with both sides mistaking each other for Thanos's soldiers before realizing they have a common enemy and joining forces. Tony and Quill bicker over who gets to make the plan, but this ends sharply when Strange emerges from a trance. He's just used the Time Stone to scan over 14 million timelines — and out of all those possibilities, they won exactly once.
Thor positions himself and his allies reluctantly reactivate the forge. The smelting process goes along perfectly but the Asgardian is enduring an unimaginable amount of pain, with the rays visibly burning him. Finally, Thor passes out from the agony and slumps onto the floor, barely clinging to life. Their efforts only managing to forge the hammer and axe portion of Stormbreaker, but not the handle, Groot uses his own wooden fingers to wrap around the pieces and force them together. With a mighty roar, he cuts off his left arm below the elbow to make the handle. Electrical arcs spark around Thor's fingers until the hammer-axe flies into his hand, instantly healing him.
Back on Earth, the Avengers meet with King T'Challa, the Black Panther, to arrange for the safe removal of the Mind Stone from Vision. With Thanos's army converging on Wakanda, the Avengers hold the line as they try to buy time to allow T'Challa's sister, Shuri, to remove the Mind Stone. They are joined by Bucky Barnes, whom T'Challa pressed back into service, and Banner also joins the fray using the Hulkbuster armor to compensate for his inability to summon the Hulk.
Proxima and Cull arrive in Wakanda. At a brief parley, Proxima claims that Glaive has died of injuries sustained in Edinburgh, and demands the Mind Stone be handed over. Steve refuses, with T'Challa insisting "Thanos will have nothing but dust and blood". Sneering that they have blood to spare, Proxima unleashes an army of savage Outriders, mindless alien monsters bred to kill, vowing to take the Mind Stone no matter what. T'Challa and Steve lead the charge against them, but the enemies' sheer numbers begin to wear even them down...
And then a Bifrost pillar slams into the battlefield, bringing with it Rocket, Groot, and Thor, Stormbreaker in hand. "BRING ME THANOS!", the God of Thunder bellows as he goes on the warpath, while the Avengers and their allies, renewed by the reinforcements, fight on harder.
However, during the battle Wanda — who's been guardian Vision — decides to join the fray to help even the odds. While she proves to be a formidable fighter, She leaves Vision vulnerable, which Proxima takes advantage of.
When Thanos arrives on Titan, he explains to Strange the reason behind his actions: Titan was once an idyllic planet, but Thanos worried that overpopulation would exhaust the world's limited resources. He made a proposal to cull the population by half to prevent this catastrophe, but was denied. Eventually, Titan fell into ruin and became a lifeless wasteland. Extrapolating that the resources of the universe were similarly finite, he sought the Infinity Stones to wipe out half of all life in existence, allowing the remaining half to go on living without fear of extinction.
Strange, Quill, Drax, Mantis, Stark, and Parker all go on the offensive. Eventually, they manage to subdue Thanos with a combination of restraints and Mantis's empathic abilities. As Parker and Stark attempt to pull off the Gauntlet, Quill demands to know where Gamora is, only for Nebula (who arrived on Titan late) to explain that she must have died for Thanos to retrieve the Soul Stone. Quill, enraged that Thanos killed the love of his life, snaps and attacks Thanos, loosening Mantis' grip seconds before Spider-Man can take the Gauntlet. Enraged, Thanos throws everything he has at the heroes, including debris from Titan's moon. After stabbing Iron Man through the abdomen with his own nano-forged blade, Thanos is given the Time Stone by Doctor Strange in exchange for Stark's life, then makes his way to Earth to take the final Stone. When Tony asks Strange why he would do that, Strange implies this is that one chance at victory he saw, and anything else is already off the table. "We're in the endgame now."
Back on Earth, the battle turns in the heroes' favor, but the attempt to remove the Mind Stone is thwarted by the very-much alive Glaive, and Vision is forced into the field. Captain Rogers intervenes at the last minute and orders Vision to flee while he fights Glaive. However the alien assassin proves too strong and almost overpowers the super-soldier, until Vision stabs him in the back with his own spear, killing him.
"I thought I told you to go." Steve says.
"We don't trade lives, Captain," he replies.
Proxima and Cull Obsidian are all killed in the ensuing brawl, and Thor has the Outriders on the run. But just as the heroes stop to catch their breath, Vision's Mind Stone seizes as a sudden chill fills the air.
"Everyone on my position," orders Steve. "We have incoming."
Thanos arrives and marches steadily towards the Mind Stone, swatting aside anyone in his way. Wanda reluctantly obliges Vision's wish and channels her energy into the Mind Stone, overpowering it and destroying it, along with Vision. After a moment of conciliation to Wanda, Thanos uses the Time Stone to undo Vision's sacrifice, re-constituting him and the Mind Stone, then tears the Mind Stone from Vision's head, killing him.
At last, Thanos has all six Infinity Stones. Thor finally arrives, blasting the Mad Titan with lightning before hurling Stormbreaker at Thanos, the blade slamming full into his chest. Reminding him of his prior vow of vengeance, he pushes the axe deeper to ensure his suffering.
Weakened, but not dead, Thanos say to the Asgardian that he "should've [aimed] for the head," raises his Gauntlet-clad hand, and with the last of his strength snaps his fingers...
In the instant after unleashing all of the Stones' power, Thanos has a vision of a young Gamora, who asks if he accomplished his goal.
"Yes," he says.
"What did it cost?" asks the young girl.
"...Everything." Thanos answers solemnly.
Back in the real world, the Gauntlet is heavily damaged, and Thanos retreats. With Thor and Steve stunned at what has transpired and wondering where Thanos has went.
"Steve..." Bucky whispers. When Steve turns to face his old friend, Bucky's body disintegrates into ash and scatters into the wind.
Soon the majority of the Wakanda forces crumble into nothingness. T'Challa goes to pull Okoye back on her feet before his body, too, disintigrates.
"I am Groot..." Groot moans before his body scatters like dead leaves in the wind before Rocket's eyes.
Wanda, the love of her life dead, stares up to the heavens before she fades away.
Rodey tries to find Sam, but to no avail; his body disintegrates before he can be found.
On Titan, as the group recovers, Mantis senses that something catastrophic has occurred, moments before she turns into ash.
"...Quill...?" Drax asks pleadingly before he fades away.
"...oh man..." Quill gasps, realization dawning in his final moments.
"Tony...there was no other way..." Strange reminds Tony before he, too, dies.
"Mr. Stark... I don't feel so good..." Peter gasps as his body also begins to crumble. Tony holds onto him tightly and tries to reassure Peter as the young boy apologizes before crumbling away in Tony's arms.
At this moment the heroes realize that Thanos has won, having completed his goal of destroying half of all life in the universe.
As Tony, Nebula, Rocket, and the surviving Avengers are left in despair with the reality of Thanos's plan succeeding, Steve turns over Vision's lifeless, colorless body, and the extent of their failure crashes down on them.
"... Oh, God." Is all Steve can say.
Thanos teleports to a far-away planet, and watches the sun set pensively as he feels a sense of relief now that his life's work is complete, and contentment with the knowledge that he was the only one in the universe with the will to do what was necessary.
In The Stinger, Nick Fury and Maria Hill are trying to figure out the situation while driving through Manhattan when the car in front of them abruptly crashes. Hill goes to check on the driver, only to find the car is empty. When a helicopter suddenly crashes into the side of a building, they realize something colossal just happened. Fury watches in horror as Hill fades away, then sprints back to his car and grabs a high-tech pager to send out a distress call, only to find himself disintegrating. "Oh, no. Motherf-..." Fury curses as he fades away.
His distress call, however, manages to go through. Its recipient: Captain Marvel.